Sealing Leaking Ponds
Excessive seepage in ponds is generally due either to a poor site--that is, one in which
the soils in the impounded area are too permeable to hold water--to improperly
constructed levees or dams, or to physical damage caused by tree roots, burrowing
rodents, or other factors. Selecting a poor site is often the result of inadequate
preliminary site investigations and is, therefore, an avoidable situation. You can avoid
improper construction just as you can avoid the establishment of trees and shrubs that
develop root systems that undermine the dam.
In places where a satisfactory site is not available, the need for water may be sufficient
to justify using the site. If so, the original pond design must include plans for reducing
seepage by sealing the pond bottom. In some places, excessive removal of topsoil
during construction, usually to provide material for the embankment, exposes highly
pervious materials such as sand, gravel, or rock containing cracks, crevices, or channels.
This is usually avoided by carefully selecting the source of embankment material.
To prevent excessive seepage, reduce the permeability of the soil to a point at which
losses are insignificant or at least tolerable. The method depends largely on the
proportions of coarse-grained sand and gravel and of fine-grained clay and silt in the
soil.
Whatever the reason your pond loses water, it is important to remember two major
considerations. First, once the pond is constructed, correcting these problems can be
costly! Second, if the seep or leak is confined to a small area, you are much more likely to
be successful in solving the problem with one of the following methods.
Compaction
Some pond areas can be made relatively impervious by compaction if the material has
different particle sizes--small gravel or coarse sand to fine sand--and enough clay (usually
10 percent or more) and silt to effect a seal. This is the least expensive method of those
presented in this publication. Compaction, however, is limited to these specific soil
conditions and by the depth of water to be impounded.
The procedure is simple. Clear the pond area of all trees and other vegetation. Fill all
stump holes, crevices, and similar areas with impervious material. Scarify the soil to a
depth of 8 to 10 inches with a disk, rototiller, pulverizer, or similar equipment. Remove
all rocks and tree roots. Roll the loosened soil under optimum moisture conditions to a
dense, tight layer with four to six passes of a sheepsfoot roller.
Make the compacted seal no less than 8 inches thick to impound areas where the depth
is 10 feet or less. Since seepage losses vary directly with the depth of water impounded
over an area, increase the thickness of the compacted seal proportionately if the depth
of water impounded exceeds 10 feet. Compact the soils in two or more layers, with each
layer not exceeding 8 inches in that section of the pond where the water depth exceeds
10 feet. Remove and stockpile the top layer or layers while the bottom layer is being
compacted.
Clay Blankets
Pond areas containing high percentages of coarse-grained soils but lacking enough clay
to prevent excessive seepage often can be sealed by blanketing. Blanket the upstream
slope of the embankment and the entire area over which water is to be impounded.
Make the blanket consist of a well-graded material containing at least 20 percent clay.
You can usually obtain material for the blanket from a borrow area close enough to the
pond to permit hauling at a reasonable cost.
Thickness of the blanket depends on the depth of the water to be impounded. The
minimum thickness is 12 inches for all depths of water up to 10 feet. Increase this
thickness by 2 inches for each foot of water more than 10 feet.
Remove all trees and other vegetation. Fill all holes and crevices before hauling earth
material from the borrow area to the pond site in tractor-pulled wheeled scrapers or
similar equipment. Spread the material uniformly over the area in layers 6 to 8 inches
thick. Compact each layer thoroughly, under optimum moisture conditions, by four to
six passes of a sheepsfoot roller before placing the next layer.
Protect clay blankets against cracking that results from drying and against rupture
caused by freezing and thawing. Spread a cover of gravel 12 to 18 inches thick over the
blanket below the anticipated high water level. Use rock riprap or other suitable
material to protect areas where the water flow into the pond is concentrated.
Bentonite
Adding bentonite is another method of reducing seepage in soils containing high
percentage of coarse-grained particles and not enough clay. Bentonite is a fine-textured
colloidal clay. When wet, it absorbs several times its own weight of water and, at
complete saturation, swells to as much as 8 to 20 times its original volume. Mixed in the
correct proportions with well-graded coarse-grained material, thoroughly compacted,
and then saturated, the particles of bentonite swell until they fill the pores to the point
the mixture is nearly impervious to water. Upon drying, however, bentonite shrinks to its
original volume resulting in cracks; therefore, sealing with bentonite is usually not
recommended for ponds in which the water level is expected to fluctuate widely. A
laboratory analysis of soil particle size distribution is required to decide the rate of
application.
Before selecting this method of sealing a pond, find the nearest source of bentonite and
investigate freight rates. If the source is far from the pond site, the cost may prohibit the
use of bentonite.
As with other methods, clear all vegetation from the area to be sealed. Fill all holes or
crevices, and use suitable fill material to cover and compact areas of exposed gravel.
The soil moisture level in the area to be treated is important. Investigate it before
applying bentonite. You need an optimum, medium to low, moisture level for good
compaction. If the area is too wet, postpone sealing until moisture conditions are
satisfactory. If it is too dry, add water by sprinkling.
Spread the bentonite carefully and uniformly over the area to be treated at the rate
determined by the laboratory analysis. This rate is usually 1 to 3 pounds per square foot
area. Thoroughly mix the bentonite with the surface soil to a depth of at least 6 inches.
A rototiller is best for this operation but you can use a disk or similar piece of equipment;
then compact the area with four to six passes of a sheepsfoot roller.
If considerable time elapses between applying the bentonite and filling the pond,
protecting the treated area against drying and cracking may be necessary. A mulch of
straw or hay pinned to the surface by the final passes of the sheepsfoot roller gives this
protection. Use rock riprap or other suitable material to protect areas where water
inflow into the treated area is concentrated.
Chemical Additives
Because of the structure or arrangement of the clay particles, seepage is often excessive
in fine-grained soils. If these particles are arranged at random with end-to-plate or endto-end contacts, they form an open, porous, or honeycombed structure, and the soil is
said to be aggregated. Applying small amounts of certain chemicals to these porous
aggregates may result in collapse of the open structure and rearrangement of the clay
particles. This dispersed structure reduces soil permeability. The chemicals used are
called dispersing agents.
The soils in the pond area need to contain more than 50 percent fine-grained material
(silt and clay) and at least 15 percent clay for chemical treatment to be effective.
Chemical treatment is not effective in coarse-grained soils.
Although many soluble salts are dispersing agents, sodium polyphosphates and sodium
chloride (common salt) are most commonly used. Of the sodium polyphosphates,
tetrasodium pyrophosphate and sodium tripolyphosphate are most effective. You can
also use a technical grade soda ash (99 to 100 percent sodium carbonate). Sodium
polyphosphates are usually applied at a rate of 0.05 to 0.10 pound per square foot and
sodium chloride at a rate of 0.20 to 0.33 pound per square foot. Soda ash is applied at a
rate of 0.10 to 0.20 pound per square foot. A laboratory analysis of the soil in the pond
area is essential to determine which dispersing agent will be most effective and to
determine the rate at which to apply it.
Mix the dispersing agent with the surface soil and then compact it to form a blanket.
Thickness of the blanket depends on the depth of water to be impounded. For water
less than 8 feet deep, the blanket should be at least 6 inches thick. For greater depths, it
should be 12 inches thick, treated in two 6-inch lifts.
The soil-moisture level in the area to be treated needs to be near the optimum level for
good compaction. If the soil is too wet, postpone treatment. Polyphosphates release
water from soil, and the material may become too wet to handle. If the soil is too dry,
add water by sprinkling or wait for a light rainfall.
Clear all vegetation and trash from the area to be treated. Cover rock outcrops and
other exposed areas of highly permeable material with 2 to 3 feet of fine-grained
material. Thoroughly compact this material. In cavernous limestone areas, the success or
failure of the seal may depend on the thickness and compaction of this initial blanket.
Apply the dispersing agent uniformly over the pond area at a rate determined by
laboratory analysis. You can apply the agent with a seeder, drill, fertilizer, spreader, or by
hand broadcasting. The dispersant should be finely granular, with at least 95 percent
passing a No. 30 sieve and less than 5 percent passing a No. 100 sieve.
Thoroughly mix the dispersing agent into each 6-inch layer to be treated. You can use a
disk, rototiller, pulverizer, or similar equipment. Operating the mixing equipment in two
directions produces best results. Thoroughly compact each chemically treated layer with
four to six passes of a sheepsfoot roller.
Protect the treated blanket against puncturing by livestock. Cover the area near the
high-water line with a 12- to 18-inch blanket of gravel or other suitable material to
protect it against erosion. Use riprap or other suitable material on areas where inflow
into the pond is concentrated.
Waterproofed Linings
Using waterproofed linings is another method of reducing excessive seepage in ponds
with coarse-grained and fine-grained soils. Polyethylene, vinyl, butyl-rubber membranes,
and asphalt-sealed fabric liners are gaining wide acceptance as linings for ponds
because they virtually eliminate seepage if properly installed.
Thin films of these materials are structurally weak, but if not broken or punctured, they
are almost completely watertight. Black polyethylene films are less expensive and have
better aging properties than vinyl. Vinyl, on the other hand, is more resistant to impact
damage and is readily seamed and patched with a solvent cement. Polyethylene can be
joined or patched with a special cement.
All plastic membranes need to have a cover of earth or earth and gravel not less than 6
inches thick to protect against punctures. Butyl-rubber membranes need not be covered
except in areas traveled by livestock. In these areas, use a minimum of 9 inches on all
types of flexible membranes. Make the bottom 3 inches of cover no coarser than silty
sand.
Clear the pond area of all undesired vegetation; fill all holes and remove roots, sharp
stones, or other objects that might puncture the film. If the material is stony or of very
coarse texture, cover it with a cushion layer of fine-textured material before placing the
lining.
Some plants may penetrate the vinyl and polyethylene films. If nutgrass, johnsongrass,
quackgrass, and other plants with high potential for liner penetration are present,
sterilizing the subgrade is desirable, especially the side slopes. Several good chemical
sterilizers are available commercially. Sterilization is not required for covered butylrubber linings 20 to 30 mils thick.
Lay the linings in sections or strips, allowing a 6-inch overlap for seaming. Make the vinyl
and butyl-rubber linings smooth but slack. Polyethylene should have up to 10 percent
slack. Be extremely careful to avoid punctures. Anchor the top of the lining by burying it
in a trench dug completely around the pond at or above the normal water level. Make
the anchor trench 8 to 10 inches deep and about 12 inches wide.
Summary
In most cases, you can avoid leaking ponds by wise site selection and proper
maintenance. In some instances, however, leaks and seeps can develop. If the leak or
seep is severe, correction is warranted, and usually will be costly.
It is a good idea to prevent trees and shrubs from becoming established on levees and
dams. Take action to remove burrowing animals such as muskrats and beavers and
nutria as soon as you notice them.
If maintenance activity does fail, however, one of the corrective measures described in
this publication might prove useful.